This invention relates generally to voltage boost circuits and, more particularly, to a circuit for shutting off an output transistor when the current flowing therethrough exceeds a predetermined value.
The voltage booster of a bubble memory operational driver requires circuitry for sensing when the voltage on an inductor decreases to a value approximately equal to the supply voltage so as to determine the earliest time at which an output pull-down transistor may be turned on which in turn permits current to flow in the inductor. An example of a voltage sense circuit for performing this function is shown and described in copending U.S. Pat. No. 4,461,989 entitled LOW CURRENT VOLTAGE BOOST CIRCUIT filed of even date herewith and assigned to the assignee of the present invention.
At some time after the pull-down transistor has been turned on, the current flowing through the inductor and therefore the current flowing through the pull-down transistor will reach a value at which the pull-down transistor must be turned off to limit the peak current, avoid saturating the pull-down transistor or inductor, and setting the switching frequency to a reasonable value (e.g. 50 kHz) without requiring an external capacitor. Since the current which is being monitored is rather high (e.g. 150 milliamperes) and since it is necessary to get the greatest drop across the inductor with only a relatively small supply voltage (e.g. 5 volts), sensing the voltage drop across a series emitter dropping resistor in order to determine current is not an acceptable technique. Furthermore, the switching point at which the pull-down transistor is turned on depends on the absolute value of the series emitter dropping resistor which may vary with temperature causing the switch point to vary with temperature.